Electrokinetic phenomena include electromigration, electroosmosis, and electrophoresis. Electroosmosis is defined as the mass flux of a fluid containing ions through a stationary porous medium caused by the application of an electrical potential. The fluid moves through the voids in the porous medium (e.g. soil) called pores. Each pore has a thin layer of charged fluid next to the pore wall having a typical thickness of between about 1 and about 10 nanometers. The thin layer of charged fluid next to the pore wall is present to neutralize the charge on the surface of the soil particle that forms the pore wall. Fluid movement occurs in soil pores because of the charge interaction between the bulk of the liquid in the pore and the thin layer of charged fluid next to the pore wall. Under the influence of a DC electric field, the thin layer of charged fluid moves in a direction parallel to the electric field. Large amounts of liquid may be transported along with the thin layer of charged fluid as well as contaminants or other species contained within the liquid.
In contrast, electromigration is defined as the mass flux of a charged ionic or polar species within a liquid or solution from one electrode to another electrode. Electromigration and electroosmosis may occur simultaneously and are the dominant mechanisms through which conventional electrokinetic transport processes occur.
Electroosmosis has been used for fifty years as a method for dewatering soils and sludges. One recent application in which electrokinetic transport of materials has found practical use is the electrokinetic remediation of contaminants in soil. Electrokinetic remediation, frequently referred to as either electrokinetic soil processing, electromigration, electrochemical decontamination or electroremediation, uses electrical currents applied across electrode pairs placed in the ground to extract radionuclides, heavy metals, certain organic compounds, or mixed inorganic and organic species from soils and slurries. The contaminants in a liquid phase in the soil are moved under the action of the electrical field to wells where the contaminants are then pumped out.
During electrokinetic processing, water in the immediate vicinity of the electrodes is electrolyzed to produce H.sup.+ ions at the anode and OH.sup.- ions at the cathode, causing the pH of the soil to change, according to the following equations.
Anode Reaction EQU 2H.sub.2 O.fwdarw.O.sub.2 +4e.sup.- +4H.sup.+ Equation (1)
Cathode Reaction EQU 2H.sub.2 O+2e.sup.-.fwdarw.H.sub.2 +2OH.sup.- Equation (2)
If the ions produced are not removed or neutralized, these reactions lower the pH at the anode and raise the pH at the cathode. Protons formed at the anode migrate towards the cathode and can aid in contaminant removal by solubilizing certain types of contaminants to form ionic species that are readily transported through the soil via electromigration or along with the charged fluid traveling in the soil pores thus increasing contaminant extraction. In contrast, the negatively charged hydroxyl ions formed at the cathode do not migrate as efficiently as protons in soil having a predominantly negative charge and can increase the soil pH in the cathode region to as high as a pH of 12. An increase of pH can cause deposition of insoluble species and precipitation of soluble species at or in the vicinity of the cathode thereby forming regions of high electrical resistivity and lowering the rate of electroosmotic flow. These types of pH changes can have a significant effect on the soil's .zeta.-potential, solubility, ionic state and charge, and the adsorption of contaminants. It is, therefore, desirable to control the pH of the fluids in the vicinity of the electrodes as well as the volume and type of fluid transported from the anode to the cathode.
The electrical charge on the surface of soil is important to the transport of liquids by electroosmosis. The charge imparted on the soil when in contact with an aqueous solution results from a number of effects, including chemical and physical adsorption and the composition of the aqueous solution and its pH. Several reports have shown that an acid front moving through the soil in the direction from anode to cathode reduces and eventually stops the electroosmotic flow. It would be beneficial to monitor and adjust the pH in the wells to impart the desired charge on the soil thus maintaining an appropriate rate of electroosmotic flow.
When current is applied through the soil, a net flux of fluid occurs by electroosmosis. This may cause a net loss of water from the vicinity of some electrodes and a net gain of fluid in the vicinity of other electrodes. It is also important to note that the rate of loss from any particular well area may be different from nearby or adjacent wells. It is unlikely in a field installation that the electroosmotic flow rate will be the same for any two wells. This is due to the fact that earth, soil and sediments, etc., are heterogeneous. Large variations in physical properties of the soil (e.g., permeability to water) may occur within short distances from a given point. Additionally, electrodes may be positioned in different locations relative to the depth of the water table which will also affect the fluid conditions around a particular electrode. In a given electrode array, the regions around some electrodes may experience high fluid losses while other electrodes in the array may experience large fluid gains. Therefore, there is a need to manage the flow of fluids into and out of the wells on individual well basis.
Another aspect of fluid management involves contaminant recovery. Contaminants present in soil tend to accumulate in the vicinity of the electrodes as well as in the electrode wells. Some metals will electroplate onto the electrode itself, however, the contaminants usually accumulate in the fluid volume surrounding the electrode and the best method for removal is to recover the fluid surrounding the electrode. This results in an overall loss of fluid from the system. It is therefore desirable to concentrate contaminants in individual wells prior to removal of the fluid in order to maximize contaminant removal and minimize the amount of fluid removed from the system.
Electroremediation processes must lend themselves to large scale field applications, since most sites in need of de-contamination are greater in size than the typical bench scale setup used for research purposes. A variety of fluid additions and removals are required to manage the needs of each individual well and remove the contaminants from the wells. Ideally, each well would have a water supply line, an acid supply line, a base supply line, and a fluid removal line. However, when a site is 30 to 50 feet square and 20-50 electrodes are required for electroremediation, having four fluid lines connected to each well is impractical to install and manage. A system that is simple in design yet allows for individual fluid management of each individual anode and cathode well would be very beneficial.